Water-rich planets: How habitable is a water layer deeper than on Earth?

Water is necessary for the origin and survival of life as we know it. In the search for life-friendly worlds, water-rich planets therefore are obvious candidates and have attracted increasing attention in recent years. The surface H₂O layer on such planets (containing a liquid water ocean and possibly high-pressure ice below a specific depth) could potentially be hundreds of kilometres deep depending on the water content and the evolution of the proto-atmosphere.We study possible constraints for the habitability of deep water layers and introduce a new habitability classification relevant for water-rich planets (from Mars-size to super-Earth-size planets). A new ocean model has been developed that is coupled to a thermal evolution model of the mantle and core. Our interior structure model takes into account depth-dependent thermodynamic properties and the possible formation of high-pressure ice.We find that heat flowing out of the silicate mantle can melt an ice layer from below (in some cases episodically), depending mainly on the thickness of the ocean-ice shell, the mass of the planet, the surface temperature and the interior parameters (e.g. radioactive mantle heat sources). The high pressure at the bottom of deep water-ice layers could also impede volcanism at the water-mantle boundary for both stagnant lid and plate tectonics silicate shells.We conclude that water-rich planets with a deep ocean, a large planet mass, a high average density or a low surface temperature are likely less habitable than planets with an Earth-like ocean.